(1) Field of the Invention
The present invention relates to a semiconductor laser diode, and more particularly to a ridge stripe type laser diode and a method for fabricating the same.
(2) Description of the Related Art
In recent years, extensive researches are in progress for attaining a higher output power and a shorter lasing wavelength in a semiconductor visible laser diode which uses GaInP or AlGaInP in the light emitting layer. An example of a laser diode which can reliably operate at more than 30 mW at 50.degree. C. has been reported in Electronics Letters, Vol. 28, No. 9 pp. 860-861, 1992. Also, an example of a short wavelength laser diode, wherein 632.7 nm cw operation was achieved at 20.degree. C., has been reported in Japanese Journal of Applied Physics (JJAP), Vol. 29, No. 9, pp. L1669-1671, 1990.
Each of the forgoing semiconductor visible laser diodes using AlGaInP layers as light emitting layers employs a structure wherein a ridged waveguide is selectively buried in a GaAs current blocking layer, and this GaAs current blocking layer causes a large absorption loss to occur with respect to an optical waveguide. This absorption loss presents a great obstacle to the achievement of low threshold currents and high-performance of semiconductor laser diodes. The inventor in the present application has experimented the use of an AlGaInP layer, which is transparent to oscillating wavelengths, in place of a GaAs layer for the current blocking layer described above.
FIG. 1A is a diagram showing a perspective view of a conventional semiconductor laser diode, and FIG. 1B shows a composition ratio of (Al+Ga)/In contained in a current blocking layer. As shown in FIG. 1A, the current blocking layer 11 is constituted by n-AlGaInP. Further, in this semiconductor laser diode, a multiquantum-well (MQW) active layer 4 is sandwiched between an upper AlGaInP cladding layer 5 and a lower AlGaInP cladding layer 3, and there are provided a double heterostructure in which the upper cladding layer 5 is formed in a mesa stripe shape and a current blocking layer 11 which selectively buries the mesa side and bottom surfaces of the upper cladding layer 5. In the drawings, the numeral 1 depicts an n-GaAs substrate, 2 depicts an n-GaAs buffer layer, 6 depicts a p-GaInP cap layer, 8 depicts a p-GaAs contact layer, 9 depicts a p-electrode and 10 depicts an n-electrode.
In the foregoing conventional semiconductor laser diode, after the cap layer 6 and the cladding layer 5 are shaped into the mesa stripe form by etching, the current blocking layer 11 of n-AlGaInP is selectively grown by an ordinary metalorganic vapor phase epitaxy method (MOVPE) without hydrogen chloride (HCl) being added.
On the other hand, as a method for selectively etching an AlGaAs layer of a material system which also includes aluminum (Al), an AlGaAs selective growth method using HCl is known. This has been disclosed by researchers of Mitsubishi Kasei Corp. in Journal of Crystal Growth, Vol. 124, pp. 235-242, 1992. This method is intended to enhance the degree of selective growth of AlGaAs by adding HCl in carrying out the MOVPE process.
However, in the conventional semiconductor laser diode shown in FIG. 1A, there is a problem in that, when the (Al+Ga)/In composition ratio in the current blocking layer 11 of the n-AlGaInP is measured by EDX (Energy Dispersive X-ray Analysis), the ratio greatly deviates from "1" at side portions of the mesa as seen in FIG. 1B. For the AlGaInP material to be lattice-mismatched with respect to the GaAs of the substrate 1, it is required that the (Al+Ga)/In composition ratio be "1". Thus, the fact that there is a large deviation from "1" in the composition ratio means that there is a large stress introduced to the sides of the mesa as a result of the lattice-mismatching.
The reason why the composition ratio deviation occurs at the sides of the mesa is thought to be probably due to diffusion coefficient differences in various materials on a selective mask. It is thought that, on the selective mask, the diffusion coefficient of each of the Al and Ga source materials is larger than that of the In source material, resulting in a large deviation of the (Al+Ga)/In composition ratio from "1" at the mesa side portions.
The composition ratio deviation explained above has a good correlation with protrusions in the current blocking layer that are formed at the sides of the mesa as seen in FIG. 1A. The material on the selective mask diffuses at the mesa side portions during the growth and the protrusions result from an increased growth rate thereof at the mesa side portions as compared with that at the flat portion.